In an accident report made public on Monday, Google disclosed that its self-driving car had caused a crash earlier this month — the first known crash caused by one of its fleet. One of the autonomous Lexus SUVs that Google has been testing on the streets of Mountain View, California, hit a bus when it tried to change lanes.

According to the account the crash was minor — the car was traveling at 2 mph when it sideswiped the public bus. However, with Google pushing to have its self-driving cars consumer-ready in the next few years, this is the first accident that finds fault with the autonomous vehicle. While Google's cars have been in accidents before, this is the first one where another driver was not at fault.

In the report, Google blamed sand bags in the road as the underlying cause for the accident. They were placed around a storm drain, and when the car detected them, it moved one lane over, hitting the bus in the process.

Tomorrow Google will release its own monthly report on the self-driving car program, in which it will address the crash. The company looks on the bright side of the incident, calling the crash "a tricky set of circumstances that’s helped us improve an important skill for navigating similar roads."

"We clearly bear some responsibility, because if our car hadn’t moved there wouldn’t have been a collision," the report reads. "That said, our test driver believed the bus was going to slow or stop to allow us to merge into the traffic, and that there would be sufficient space to do that."

Brendan Klinkenberg is a tech reporter for BuzzFeed News and is based in San Francisco.